current research
Fate of Phosphorus in Waste Treatment Processes: Enhanced Removal of Phosphate by Activated Sludge Arnold B. Menar and David Jenkins Sanitary Engineering Research Laboratory, University of California, Berkeley, Richmond, Calif. 94804
rn The activated sludge process achieves about 20% phosphorus removal from sewage containing 10 mg. P per liter to produce an activated sludge containing 2-3% P. Greater P removals and higher sludge P contents are caused by chemical precipitation, specifically, in hard water sewages by the precipitation of calcium phosphates which become entrapped in the activated sludge floc matrix. Precipitation depends on sewage pH, which depends on activated sludge COz production. High mixed liquor C 0 2 production causes low mixed liquor pH. As the mixed liquor proceeds down the aeration basin, the amount of degradable organics decrease and, in turn, both DO consumption and COn production rates decrease. At a constant air supply rate, mixed liquor DO concentration increases and C 0 2 content decreases, thus the pH of the mixed liquor increases. It is only fortuitous that at some point along the length of the aeration basin a rise in both DO and pH occurs. Operational parameters such as organic loading, mixed liquor suspended solids, and DO have no effect on the enhanced removal of phosphate by activated sludge.
I
n the considerable body of literature concerning the fate of phosphate in the activated sludge process, there appear to be two distinct schools of thought. The first viewpoint, represented in papers by Sawyer (1944), Sekikawa, Nichikawa, et al. (1966), Hall and Engelbrecht (1967)) and Jenkins and Menar (1967) can be summarized as follows: The volatile mass (VSS) of activated sludge contains between 2 and 3 by weight of biologically incorporated phosphorus; the amount of phosphate incorporated biologically into activated sludge is not affected by the growth rate of the sludge or by process operating parameters such as organic loading, mixed liquor volatile suspended solids concentration (MLVSS), aeration rate, or mixed liquor dissolved oxygen (DO) concentrations; and phosphate removal is proportional to the net sludge growth. With activated sludge P contents of 2 to 3%, domestic sewage P contents of 10 mg. P per liter and substrate removal rates in the standard rate range (0.4 to 1.0 lb. COD removed per lb. MLVSS-day), biological removal of phosphate can only account for a maximum of 20 to 30% of the influent phosphate or about 2 to 3 mg. of P per liter of an influent 10 mg. of P per liter.
The second point of view, expounded in papers by Levin and Shapiro (1965)) Borchardt and Azad (1967), and Connell and Vacker, (1967), embodies the idea that under certain conditions, activated sludge is capable of removing more than the 2 to 3 % phosphate-P that it requires for growth. The biological uptake of excess phosphate has been termed “luxury uptake.” According to these authors, phosphate removal by this mechanism is enhanced by high-DO concentrations and, conversely, phosphate incorporated into sludges by the luxury uptake mechanism can be released easily under anoxic conditions and conditions of low pH, such as those that might be encountered in the sludge blanket at the bottom of a secondary clarifier or in an anaerobic digestor. Further, luxury uptake of phosphate leads to sludges with phosphate contents two to three times greater than normal sludges, and consequently, treatment of domestic sewage at standard organic loading rates can lead to phosphate removals of 60 to 90 of an influent 10 mg. P per liter. A mechanism for the enhanced removal of phosphate must be consistent with these observations and, in addition, must provide satisfactory interpretation of the following observations made at the Rilling Road Plant, San Antonio, Tex.-a plant that has experienced phosphate removals as high as 90 %. At a point about 3/4 of the way along the 300-ft. aeration basin where the DO concentration increases from about 1.2 to 5 mg./liter, there is a sharp drop in soluble orthophosphate concentration. “Reversed tapered aeration,” in which higher aeration rates are used at the tail end of the aeration basin than the head end of the basin, enhanced the removal of phosphate. During aeration at the Rilling Road Plant, the pH of the sewage increased from 7.3 to 7.9. Activated sludge with a high phosphate content had a very low (approximately 7 0 z ) volatile matter fraction.
z
Postulated Mechanism of Enhanced Phosphate Removal Phosphate removal in excess of that predicted by biological growth requirements is caused by chemical precipitation. The phosphate precipitate is physically entrapped in the matrix of the activated sludge floc and is removed with the wasteactivated sludge. In hard water sewages, such as that at San Antonio, Tex., a significant phosphate removal is by calcium phosphate precipitation-the solubility of the calcium phosphate being a function of the pH of the sewage. In the metabolism of organic matter activated sludge conVolume 4, Number 12, December 1970 1115
Table I. Comparison of Mineral Characteristics of Water from San Antonio, Tex., and Sewage from San Ramon, Calif. San Antonio San Ramon Constituent mg./liter rng./iiter
PO4 as P Ca Mg Total hardness as CaC03 Alkalinity as CaC03 PH 0
9b 64' 17" 230' 225b 7.30
16 66 44 346 405 7.7
Durfor and Becker, 1964. 1967.
* Vacker.
sumes oxygen and produces C02, which controls the mixed liquor pH. COa concentrations of 0.5 by volume (compared to typical atmospheric CO1 contents of 0 . 0 3 z by volume) have been determined by Weddle (1968) in the diffused air issuing from an activated sludge aeration basin. In an activated sludge aeration basin the DO concentration is controlled by the rate of aeration and the rate at which the activated sludge consumes oxygen (for a constant rate of oxygen transfer). Since Onis consumed and CO1 is produced by activated sludge in the metabolism of its substrate, the rates of O2uptake and C o nproduction should parallel each other. In a conventionally operated activated sludge basin, return activated sludge and settled sewage enter at the head end of the basin. Here, the rates of COzproduction and DO consumption are high and, consequently, the mixed liquor DO concentration is low. A high CO1 production rate leads to high mixed liquor COz concentrations and a low mixed liquor pH. As the mixed liquor proceeds down the aeration basin, the amount of degradable organic matter decreases ; the DO consumption and C o nproduction rates decrease, the mixed liquor DO concentration increases ; and the dissolved CO1 content decreases and the mixed liquor pH rises. Thus, an increase in DO concentration should be accompanied by an increase in mixed liquor pH to a value that, at some point along the length of the basin, calcium phosphate precipitation will occur. This point will coincide fortuitously with the point at which the mixed liquor DO concentration increases, and significantly with the point at which a decreased mixed liquor CO1 content has allowed the pH to rise to a point where calcium phosphate precipitation occurs. Several other changes that take place during aeration may influence the precipitation of calcium phosphate. In settled sewage, the total phosphate consists of dissolved orthophosphate, some organic phosphate, both in solution and suspension, and condensed phosphates. Calcium may be present as Ca2+and also as particulate salts with fatty acid anions and particulate or dissolved complexes with orthophosphate condensed phosphate anions. Activated sludge treatment rapidly hydrolyzes condensed phosphates to orthophosphate, possibly releasing calcium from condensed phosphate complexes. Microbial degradation of fatty acid salts of calcium would release Ca?+ions and degradation of organic compounds might release small amounts of phosphate, The net result of these processes is that both the postulated precipitating species-orthophosphate and calcium-would increase during activated sludge treatment. Anoxic conditions develop in activated sludge when the DO consumption rate exceeds the oxygen supply rate. Since DO consumption is accompanied by CO1 production, anoxic conditions should be accompanied by a pH decrease. The pH
z
1116 Environmental Science & Technology
decrease should be more severe in the activated sludge floc matrix than in the liquid surrounding the floc, especially in a quiescent sludge blanket. Because of the intimate association of the colloidal calcium phosphate particles with the activated sludge matrix, pH decreases associated with anoxic conditions will be especially felt by the particles and it is possible that in an anoxic floc interior locally acid conditions would cause calcium phosphate dissolution and release of orthophosphate into solution. It has been observed that phosphate is released from activated sludge when the DO concentration is below 0.3 mg./ liter (Levin and Shapiro, 1965) and that the released phosphate is from the acid-soluble fraction (Shapiro, 1967). An enhanced phosphate removal mechanism involving calcium phosphate precipitation is therefore plausible in light of reported observations. This paper will provide experimental evidence that confirms the precipitation hypothesis. Experimental Effect of Aerating Gas on Calcium Phosphate Precipitation. Batches of activated sludge effluent from the Valley Community Services District (VCSD) Water Reclamation Plant, San Ramon, Calif., which has mineral characteristics similar but not identical to those of San Antonio sewage (Table I), were aerated with air, pure nitrogen, pure oxygen, and air 5 COz. The results of this experiment (Figure 1) show that the important function of the aerating gas is COn-stripping, which subsequently causes an increase in the pH of the liquid. DO concentration per se is unimportant in the removal of dissolved orthophosphate, since O2 and NZwere equally effective in causing calcium phosphate precipitation. These two gases raised the pH of the activated sludge effluent to an identical value and at an identical rate, while the DO in the effluent aerated with O2 reached 34.4 mg./liter, the DO in the sample bubbled with Nzfell from 7.2 to 1.7 mg./liter.
+
z
E 20
4 E
W 0 ?I
s5
z-
0
1 7 5 1
u? w 0
1
2
3
4
50.751
1
1
1
I
I
0
1
2
3
4
5
5
TIME OF AERATION, hours
1
LEGEND AIR 0
AIR
+
5 % C02
A
OXYGEN
0
NITROGEN
Figure 1. Batch aeration of activated sludge effluentwith air, air COS,02, and NZ
5
+
Table 11. Release of Orthophosphate from Activated Sludge Initial After 3-hr. aeration with Air concenAir Analysis tration Air 0.5% COS5 % C 0 2
DO, mg./liter PH Dissolved orthophosphate, mM/liter Dissolved calcium mM/liter Mole ratio of released calcium and orthophosphate: Ca:P
+
+
5.8 8.17
8.4 8.43
8.4 7.60
8.4 6.85
0.33
0.25
0.51
1.04
1.49
1.36
1.72
2.45
...
...
1.28
1.35
Air was not as effective as either N2or 0, in causing the precipitation of orthophosphate because air, containing approximately 0.03 C o n ,is not as efficient a C02-stripperas pure N1 or 0,. This is reflected by the pH changes caused by bubbling air, in comparison to those caused by Nz and 0,.The final pH in the N P -and 02-bubbled activated sludge effluents was 9.20, whereas the final pH of the effluent bubbled with air was 8.65. Similarly, dissolved orthophosphate and calcium levels fell more rapidly and to lower values in the N2- and 02-aerated samples than in the effluent bubbled with air. Aeration with a i r + 5 x COz caused a pH decrease from 7.75 to 6.95, since the aerating gas mixture contained a higher CO, content than that to which the activated sludge effluent had been equilibrated. A small increase in dissolved orthophosphate and calcium took place, indicating that the sludge contained some calcium and orthophosphate that could be solubilized at a pH below 7.75. Comparison of the results of aeration with air and with air + 5 % CO, provides further indication of the unimportance of DO concentration in orthophosphate precipitation. While the D O concentration in both samples was identical at all times during the experiment, the dissolved orthophosphate concentration fell from 14.6 to 9.4 mg. P per liter in the air-bubbled sample and rose from 14.6 to 16.9 mg. P per liter in the sample aerated with air+5% C 0 2 . Release of Calcium Phosphate from Activated Sludge. Aliquots of activated sludge mixed liquor from a pilot plant that was being operated to produce a high phosphate sludge (5.2 % P on a VSS basis) were bubbled with air, airS0.5 % CO,, and air+5 % CO,.After 3 hr., the DO concentration in all three aliquots increased from 5.8 to 8.4 mg./liter (Table 11). In the sample aerated with air the pH increased from 8.17 to 8.43, and the dissolved calcium and orthophosphate decreased by 0.1 3 and 0.08 mM/liter, respectively. In the activated sludges aerated with air+0.5 Z CO, and with air + 5 %Con,the pH fell from 8.17 to 7.60 and 6.85, respectively. In both of these samples, dissolved calcium and orthophosphate were released (Table II), the releases being greater for the sample aerated with air $ 5 % C o n(and having the lower pH). Pilot Plant Experiments. A four-compartment, 300-liter aeration-capacity activated sludge pilot plant was located at the VCSD Water Reclamation Plant, San Ramon, Calif. The pilot plant was fed with primary effluent from the VCSD primary sedimentation basins. Samples of the influent and effluent flows were taken automatically every 15 min. and composited by pumping into refrigerated containers. During each experiment, the pilot-plant performance (and where applicable, the performance of the VCSD plant)
was characterized by organic loading, hydraulic residence time, mean cell residence time, and substrate removal efficiency. For each experimental period the daily measurements of process performance were graphically summarized, and periods of steady operation were determined by inspection. The criterion for steady-state operation of the pilot activated sludge plant was *30% deviation of daily values of MLVSS and substrate removal rate from the mean value and a &15% deviation of daily sludge Ca and P values from the mean value. The data during periods of steady-state operation were then screened for conformance to a phosphate materials balance. The criterion for conformance to the phosphate materials balance was * l o % from completion. Only data that conformed to steady-state and phosphate materials balance criteria were used. San Antonio Conditions. The pilot plant was operated to conform to the parameters reported to be important for enhanced phosphate removal by Connell and Vacker (1967) in their studies at the San Antonio Rilling Road Plant. The pilot plant aeration basin configuration with four 75-liter continuously stirred tank reactors (CSTR) in series and with settled sewage and return activated sludge entering at the first reactor, closely resembled the two-pass aeration basin of the Rilling Road Treatment Plant. The results of the six-week experiment (Table 111) show that enhanced phosphate removal was not obtained in the pilot plant when the San Antonio conditions were matched almost identically. Thus, under conditions of identical MLVSS, BOD loading, effluent N03-N, and at mixed liquor DO concentrations that consistently exceeded the specified minimum of 1.5 mg./liter, the pilot plant reduced the phosphate concentration from 15.2 to 12.2 mg. P per litera removal of phosphate that could be accounted for entirely by incorporation into activated sludge cell material-Le., biological removal for growth. Indeed, during the entire period of operation under San Antonio conditions, the activated sludge P content in the pilot plant never exceeded 2.9 % on a volatile matter basis and averaged 2.3 P-a figure which is in excellent agreement with the 2.6% P obtained in previous work (Jenkins and Menar, 1967). It was concluded that the conditions of MLSS, BOD loading, effluent N03-N, and mixed liquor DO reported by Conne11 and Vacker (1967) were of no direct significance in the enhanced phosphate removal by activated sludge in this study.
Table 111. Comparison of Average Operating Conditions and Phosphate Removal at San Antonio Rilling Road Plant and at Pilot Plant Operated under San Antonio Conditions Rilling Road= Pilot Parameter Plant plant
MLSS, mg./liter BOD loading, lb. BODjlb. MLSS-day Mean cell residence time, days (sludge age) Mixed liquor DO, mg./liter Effluent NO8-N, mg. N/liter Influent pH Effluent pH Influent total phosphate, mg. P/liter Effluent dissolved phosphate, mg. P/liter Phosphate removed, mg. P/liter Q
1130 0.5
1130 0.5
3.2 3.0 1.0 7.3 7.9 10.5
2.2 2.8 1.5 7.7 7.8 15.2
0.9 9.6
12.2 3.0
Connell and Vacker (1967).
Volume 4, Number 12, December 1970 1117
Table IV. Operating Conditions and Results of Pilot-Plant Preaeration Experiment Overall (preaeration Influent Preaeration activated sludge)
Operating conditions Substrate removal rate, lb. COD removedilb. MLVSS-day Mean cell residence time, days Hydraulic residence time, hr. Air supply rate, cu. ft./gal. waste treated MLSS, mg./liter MLSS, volatile fraction, Results of operation Influent total COD, mg./liter Effluent dissolved COD, mg./liter COD removal efficiency, Influent total P, mg./liter Effluent dissolved P, mg./liter MLVSS, P content, Influent total Ca, mg./liter Effluent dissolved Ca, mg./liter Influent pH Effluent pH Effluent DO, mg./liter
... ...
... ... ... ...
z
217 ...
z
Table V. Operating Conditions and Results Comparative Study Pilot plant Operating conditions Substrate removal rate, lb. COD 0.25 removed/lb. MLVSS-day 12.6 Mean cell residence time, days 7.8 Hydraulic residence time, hr. Air supply rate, cu. ft./gal. waste 97 treated MLSS, mg./liter 2780 79.1 MLSS, volatile fraction, 68.4 Mixed liquor temperature, O F . Results of operation Influent total COD, mg./liter 267 Effluent dissolved COD, mg./liter 30 88.8 COD removal efficiency, Influent total P, mg./liter. 16.6 Effluent dissolved P, mg./liter 11.8 4.0 MLVSS, P content, 69.6 Influent total Ca, mg. Ca/liter 56.0 Effluent dissolved Ca, mg. Ca/liter 4.4 MLVSS, Ca content, 7.7 Influent pH 8.3 Effluent pH 8.4 Effluent DO, mg./liter
z
z
z
z
1118 Environmental Science & Technology
...
...
62.0
7.7 ... 1.5
...
...
8.3 5.4
8.3 6.2
...
for VCSD
plant
baffled to prevent short-circuiting by dividing each reactor into four compartments. The operating conditions and the results of the three-week preaeration experiment are summarized in Table IV. With average preaeration and aeration times of 4.3 and 4.4 hr., respectively, the pH of the influent settled sewage was raised from 7.7 to 8.3 during preaeration, a pH that was maintained in the mixed liquor aeration basins. The phosphate concentration was reduced by 4.3 mg. P per liter (total influent P-dissolved effluent P) during preaeration and further reduced by 0.9 mg. P per liter by the activated sludge section of the plant. The overall reduction of 5.2 mg. P per liter is far greater than the approximately 2-3 mg. P per liter reduction that would be predicted by biological removal alone at a substrate removal rate of 0.8 lb. COD/lb. VSS-day (Jenkins and Menar, 1967). The activated sludge in the aeration basin contained 4.5 P on a VSS basis, a figure significantly greater than the average 2 . 6 z found by Jenkins and Menar (1967) for activated sludge. Calcium concentrations were reduced by 2.4 mg. Caper liter (total influent-dissolved effluent) by preaeration and by 5.6 mg. Ca per liter after aeration. From these data, it could be reasoned that, as the pH rises and as calcium and orthophosphate are released during preaeration, precipitation of a colloidal calcium phosphate takes place. On contact with the mixed liquor in the aeration basin, the colloidal calcium phosphate particles agglomerate with activated sludge and are removed. Comparative Study with VCSD Plant. The objective of the comparative study was to show that the phosphate removal of two activated sludge plants operated with identical flow patterns and organic loadings, and treating the same sewage could be regulated by the pH of the mixed liquor. The pilot plant was operated identically to the VCSD treatment facilityLe., as a sludge reaeration plant with 2 5 z of the aeration basin used for sludge reaeration. The only operating variable that differed between the two plants was the air-to-liquid ratio, which was 3 cu. ft./gal. in the VCSD plant and approximately 97 cu. ft./gal. in the pilot plant. Mean operating conditions and results of the 11-week com-
z
0.29 12.9 9.1 3.0 2935 85.2 67.5 264 23 91.3 16.8 14.1 2.1 68.0 58.0 1.4 7.7 7.2 4.2
78 64 ... 11 .o 3.2
64.4
15.3 ...
Preaeration Experiment. The purpose of this experiment was to determine whether the pH of the incoming sewage could be raised by preaeration to such a degree that phosphate precipitation would take place in the activated sludge aeration basins. The pilot plant was divided into two sections of approximately equal volume, the first two reactors serving as preaeration basins and the latter two as the activated sludge aeration ba.sins. Both the preaeration and aeration sections were further
0.8 2.3 8.7 28.0 770 76.5 41 81 ... 10.1 4.5 ... 58.8
...
z
2.4 0.18 4.3 14.0 443 81.1
+
parative study (Table V) show that the pilot plant and VCSD plant operating parameters were identical, with the minor exception that the hydraulic residence time in the pilot plant was 7.8 hr. compared with 9.1 hr. in the VCSD plant. In the VCSD plant the pH of the sewage fell from 7.7 to 7.2 during activated sludge treatment, while the increased rates of aeration in the pilot plant caused a pH increase of from 7.7 to 8.3 during treatment. The VCSD activated sludge plant removed 2.7 mg. P per liter to produce an activated sludge with 2.1 % P (VSS basis)-figures entirely predictable on the basis of phosphorus requirements for biological growth (Jenkins and Menar, 1967).The pilot plant, however, effected a removal of 4.8 mg. P per liter and produced an activated sludge containing 4.0 P (VSS basis)-values far in excess of those predictable from sludge growth requirements. The pilot plant activated sludge was significantly lower in volatile matter content than the VCSD sludge (79 compared with 85%), indicating that it contained a higher content of inorganic material. The calcium content of the pilot plant activated sludge was 4.4 Ca (VSS basis) compared with 1.4 Ca (VSS basis) in the VCSD activated sludge. The calcium removal effected by the VCSD plant was 10 mg. Ca per liter, whereas in the pilot plant 13.6 mg. Ca per liter was removed. Two additional pilot plant experiments, the first conducted at an average substrate removal rate of 0.2 lb. COD/lb. VSS/ day and MLSS of 4950 mg./liter and the second at an average substrate removal rate of 0.59 lb. CODjlb. VSSjday and MLSS of 1850 mg./liter demonstrated that all the conditions associated with enhanced phosphate removal in the previous pilotscale experiments were effective in low-rate, high-solids units and in standard rate activated sludge units (Table VI). Parallel between Ca and P Removal. During the comparative study a cold spell caused the performance of the relatively
z
x
z
x
Table VI. Operating Conditions and Results for Low LoadingHigh Solids, and Standard Rate Activated Sludge Studies Low loading-high Standard solids rate system system Operating conditions Substrate removal rate, lb. COD 0.59 removed/lb. MLVSS-day 0.2 14.5 Mean cell residence time, days 4.0 Hydraulic residence time, hr. 6.0 6.7 Air supply rate, cu. ft./gal. waste treated 83 75 MLSS, mg./liter 4950 1850 MLSS, volatile fraction, 66.1 72.6 Mixed liquor temperature, O F. 75.9 77.6 Results of operation Influent total COD, mg./liter 273 232 Effluent dissolved COD, mg./liter 33 36 87.7 84.3 COD removal efficiency, Influent total P, mg./liter 17.7 15.8 Effluent dissolved P, mg./liter 11.1 10.4 MLVSS, P content, 6.8 6.0 62.8 Influent total Ca, mg./liter 64.3 56.0 Effluent dissolved Ca, mg./liter 55.5 10.0 8.3 MLVSS, Ca content, % 7.7 7.5 Influent pH 8.1 8.1 Effluent pH 6.2 6.0 Effluent DO, mg./liter
exposed pilot plant to deteriorate, especially with regard to phosphate removal. Figure 2 shows that the phosphate and calcium contents fell simultaneously at the onset of the cold weather and recovered in perfect step following the return of normal temperatures. The parallel behavior of calcium and phosphate removals and sludge calcium and phosphate contents is consistent with the precipitation mechanism postulated for enhanced phosphate removal by the pilot plant. Discussion Laboratory and pilot plant experiments have shown that enhanced phosphate removal by activated sludge in hard water sewages can be rationally explained by calcium phosphate precipitation followed by trapping of the precipitate by the activated sludge floc. Biological uptake of phosphate by activated sludge treating domestic sewage produced an activated sludge with 2 to 3 x P on a VSS basis and results in a phosphate removal of 20 to 3 0 z of an influent 10 mg. P per liter. Laboratory experiments demonstrated the insignificance of DO as a parameter in calcium phosphate precipitation and showed that pH, which in an activated sludge aeration basin is largely controlled by the CO, content of the mixed liquor, was of decisive significance. The pilot plant experiments show the aeration basin DO fortuitously reflects the pH value of the mixed liquor (Figure 3) with high DO concentrations being associated with high pH values for a given set of operating conditions. Pilot plant activated sludge experiments have shown that decreases of dissolved phosphate were accompanied by decreases of dissolved calcium and that the activated sludge phosphate and calcium contents paralleled each other (Figure 4). Further evidence that increased activated sludge phosphate
"J
, i 48
z x
TIME
FROM START, days
Figure 2. Parallel between Ca and P removal and Ca and P in sludge for pilot plant Volume 4, Number 12, December 1970 1119
9.0
I
I
I
I
I
I
-
I
8.6 -
8.2
0 0
h -
0 0 0 0
0
7.8 -
0 0
0
0
0
8
8,'
0
0
0
7.4
OF
8 a"048b
e
0
'
-
0
Figure 3. Illustration that mixed % liquor DO reflects mixed liquor pH in pilot plant experiments
0
I
-
0
-
go 0
-
-
7.0
I
I
I
I
I
I
I
1
7 t
l
o
I
O 1
2
3
4
5
30
20
ACTIVATED SLUDGE PHOSPHATE C O N T E N T , % P OF DRY SOLIDS
50
4 0
P CONTENT
60
70
OF V S S , %
PILOT P L A N T EXPERIMENTS h
0
LOW
A
COMPARATIVE STUDY, VCSD P L A N T
A
COMPARATIVE STUDY, PILOT P L A N T
OTHER
PILOT P L A N T CONDITIONS
STANDARD RATE RATE
A A
SERL ACTIVATED SLUDGE P L A N T AND GOLDEN GATE PARK ACTIVATED SLUDGE P L A N T
I II
II
'I
TREATMENT
COMPARATIVE COMPARATIVE
0 LOW R N E TYPICAL
DATA 0
0
PILOT P L A N T PREAERATION
'I
0
SAN ANTONIO
STANDARD RATE
SERL SAN ANTONIO, APRIL 12, 1966
O # #
I,
"
29,
I'
Figure 4. Correlation of activated sludge phosphate and calcium contents
Figure 5. Correlation of activated sludge phosphate content and volatile fraction
content was associated with an inorganic compound and not caused by a biological uptake was obtained from the relationship of activated sludge phosphate content to the volatile fraction of activated sludge (Figure 5). As the phosphate content of the activated sludge increases, its volatile fraction decreases. Data included in Figure 5 from the current pilot plant studies, from the San Antonio Rilling Road Plant and from prior studies by Jenkins and Menar (1967), show that as phosphate incorporation into activated sludge increases, a progressively more nonvolatile sludge is produced. An examination of all laboratory and pilot plant data shows that there is an empirical correlation between the concentration product (Ca*+)(orthophosphate) and pH (Figure 6). While this correlation at the present time is empirical it does provide a consistent representation of the data and confirms the hypothesis that the solubility of calcium phosphate in sewage systems is a function of pH. Several activated sludge treatment operating parameters
claimed to be necessary for enhanced phosphate removal (Connell and Vacker, 1967; Levin and Shapiro, 1965) do not appear to be significant. High DO concentrations per se did not cause enhanced phosphate removal in these experiments. Connell and Vacker (1967) report that the particular organic loading of 0.5 lb. BOD/lb. MLSS-day and the particular MLSS concentration of 1000 mg./liter were most favorable for enhanced phosphate removal. Pilot plant activated sludge experiments showed that enhanced phosphate removals could occur at a wide range of organic loadings and MLSS concentrations. Other correlations of Vacker, Connell, et al. (1967) relating enhanced removal of phosphate to sludge volume index, effluent nitrate content, effluent SS concentration, and the effluent ammonia concentration were not substantiated. The involvement of a precipitation mechanism in the enhanced removal of phosphate by activated sludge has been demonstrated by this research. In hard water sewages such as those of San Antonio and VCSD, the precipitation appears to
1120 Environmental Science & Technology
16’
I
I
I
be calcium phosphate. However, it should be noted that the residual dissolved orthophosphate concentrations obtained at San Antonio at the pH values reported by Connell and Vacker (1967) for this plant are much lower than would be predicted by the correlation curve in Figure 6. Moreover, the dissolved orthophosphate residuals obtained in the pilot plant experiments at no time approached those low values of
